The measurement of the permittivity of high-loss polar liquids has historically proven to be a difficult task. Generally, the measurement of permittivity involves setting up a complex mechanical apparatus with very tightly controlled tolerances, carefully calibrating out electrical delays, and then inserting the specimen or sample. This is a time consuming process which is not readily adaptable to automation or a wide range of materials. Moreover it is susceptible to drifts in temperature of the sample, which degrade accuracy.
The measurement of the permittivity of aqueous solutions is difficult because of the high relative permittivity of water. One method for measuring permittivity is Time Domain Reflectrometry (TDR). In TDR, a fast rise time pulse is transmitted down a coaxial line until it intersects a test sample. The sample represents a discontinuity in the transmission line which causes a reflection. Measurement of the shape of the reflected pulse yields information which is a function of the reflection coefficient. From transmission line theory, the reflection coefficient is related to the characteristic impedance of the transmission line, from which the relative permittivity can be determined.
Although TDR is a widely used method for permittivity measurements (though not for aqueous solutions), it has numerous shortcomings. The sample-probe interface, for example, must have very close mechanical tolerances. As a result, TDR generally yields an accuracy of no better than two or three percent.
Another method of measuring permittivity is the Transmission Line Method (TLM) with a Network Analyzer (NWA). TLM is similar to TDR except that the NWA directly measures the reflection or transmission coefficient instead of deriving the coefficient from the shape of the waveforms. TLM is somewhat easier than TDR, however the same tight mechanical tolerances are required. With a large amount of fine tuning, accuracy of two to three percent is possible. Additionally, it is difficult to contain an aqueous sample in the transmission line without introducing spurious effects which degrade accuracy.
Most broadband permittivity measurements are made with the coaxial open-ended probe method. This is an ideal method for measuring the permittivity of polar liquids. A coaxial probe is simply placed against a sample, or in some cases into the liquid, and the changes in the reflection coefficient are measured by a NWA. These changes in reflection coefficient are measured as changes in the magnitude of the reflection coefficient and changes in the phase of the reflected signal. The sample permittivity can be calculated from the magnitude and phase of the reflection coefficient. However, the usefulness of this method is also limited to an accuracy of no better than one percent.
Many different types of coaxial open-ended probes are available. A summary of some of these techniques is found in an article by M. A. Stuchly and S. S. Stuchly, "Coaxial Line Reflection Methods for Measuring Dielectric Properties of Biological Substances at Radio and Microwave Frequencies--A Review," IEEE Trans. Instru. Meas., Vol. IM-29, no. 3, pp. 176-83, Sept. 1980.
The apparatus and method of the present invention provides a novel, highly accurate method of measuring the permittivity of aqueous solutions. Additionally, the present invention utilizes the concept of identification of compounds by measuring relaxation frequency using highly precise permittivity measurements. Large and complex molecules have low relaxation frequencies and small simple molecules have high relaxation frequencies. Therefore, if a solution has a mixture of molecules or compounds which have widely varying relaxation frequencies, and if those relaxation frequencies can be accurately measured, then the concentration of those different compounds in the solution can be determined.